Making MOFs magnetic: To extract chlorophenols

Ezine

Published: Jun 20, 2016

Author: Jon Evans

Channels: Sample Preparation

Magnetic joints

The highly porous materials known as metal-organic frameworks (MOFs) have already proven very effective for conducting solid-phase extraction (SPE). But they could potentially become even better if they could be made magnetic, as this would offer an easy way to separate them and the analytes of interest from liquid samples. The question, though, is how best to do this.

Scientists have experimented with incorporating magnetic nanoparticles into already-synthesized MOFs, but the nanoparticles are prone to blocking the all-important pores. They have also tried synthesizing MOFs directly onto magnetic nanoparticles, but this process has proved difficult to control.

Recently, however, scientists have come up with a highly promising approach that takes advantage of the MOFs inherent chemical structure. For MOFs are made up of metal-containing molecules acting as ‘joints’ linked together by long, rigid organic groups acting as ‘struts’, and so the idea is to employ joints that contain naturally magnetic metals such as iron, nickel or cobalt. Such magnetic molecules can’t be used to produce MOFs on their own, but they can be used as a minor partner in conjunction with molecules containing metals such as zinc that are more normally used to produce MOFs.

Zinc and cobalt

Chun Wang and his colleagues at the Agricultural University of Hebei in China have already used this approach to produce a magnetic version of a type of MOF known as a zeolitic imidazolate framework (ZIF). This involved using a cobalt-based molecule as the magnetic component and then heating the ZIF in the absence of oxygen to convert it into a carbon-rich form that could be used for extracting organic molecules. The resultant carbonized magnetic ZIF made a highly effective material for extracting herbicides and insecticides from water.

Now, Wang and his colleagues have used the same technique to produce a ZIF able to extract chlorophenols. These industrial chemicals are widely used in the production of pesticides and disinfectants, and so are common environmental pollutants. Again, they used a cobalt-based molecule as the magnetic component, but they combined it with a zinc-based molecule to produce a different ZIF known as ZIF-8. Without the cobalt, ZIF-8 is already produced commercially for applications such as gas separation and catalysis, which should make it easier to bring a magnetic version to the market.

Wang and his colleagues experimented with various different ratios of zinc to cobalt, although always with cobalt as the minor component, in order to determine how varying the ratio affected the surface area and magnetism of the ZIF. They found that a ratio of seven-to-one produced a version of ZIF-8 with the highest surface area while maintaining a reasonable level of magnetism. Finally, they carbonized this ZIF-8 by heating it at 900°C for six hours.

Honey tea

Once they had the magnetic, carbonized ZIF-8, they determined the optimum conditions for extracting chlorophenols from liquid samples and then subsequently releasing them from the ZIF. Chlorophenols normally possess a charge that varies in accordance with pH, and Wang and his colleagues found that their ZIF-8 was most effective at absorbing chlorophenols in acidic conditions, below a pH of around six. In keeping with this finding, they also found that alkaline methanol was most effective at releasing the chlorophenols.

As a first test of their ZIF-8, they used it to extract four chlorophenols spiked into samples of water and tea for analysis by high-performance liquid chromatography (HPLC). They found that their ZIF-8 could easily be separated from the water and tea using an external magnet and that it took all the chlorophenols with it, allowing them to be detected at levels as low as 0.1–0.2ng/mL. Furthermore, the same ZIF-8 still worked perfectly well after being used more than 10 times, with just washing in between.

Finally, they used the same approach to analyze unspiked samples of tap water and tea. While they didn’t find any chlorophenols in the tap water, they detected two different chlorophenols in the tea at concentrations of 1.1–1.2ng/mL.

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